The present invention relates to a disc drive storage system. More particularly, the present invention relates to a thermal asperity sensing head design which provides characteristic information for thermal asperity defects on a surface of a magnetic data storage disc. Disc drive data storage devices are known which read and write data from a thin layer of magnetizable material on a surface of one or more rotating discs. Read and write operations are performed through a transducer which is carried in a slider body. Slider and transducer are sometimes collectively referred to as a head. Each disc surface has a single head associated therewith to read and write data from the disc surface.
Heads are supported via an actuator assembly, which moves the heads for alignment relative to concentric data tracks on a disc surface. The actuator assembly is controlled by electronic circuitry coupled to the actuator assembly in a known manner. The head is designed to fly above the disc surface for operation via cooperation of the rotating discs and an air bearing surface on the slider. As the disc rotates, the disc drags air beneath the air bearing surface of the slider, which develops a lifting force, causing the head to lift and fly above the disc surface.
The entire disc surface of a magnetic disc is not ideal for reading and writing data. In particular, disc surfaces have asperities which interfere with the flying characteristics of the data head, as well as the read and write operations of the data head. In operation, the head can come into contact with asperities while the head flies above the surface of the disc. Potentially, this undesirable contact can cause data written to a particular location on a disc to be lost.
For example, in a magnetoresistive (MR) head which incorporates a MR type sensor, after contact with an asperity, the heat generated by the contact changes the resistive properties of the MR sensor. As a result, the corresponding signal read by the MR head is distorted by a voltage spike and subsequent decay, sometimes causing the data stored near the asperity to be unrecoverable. The voltage spike in the read signal is frequently referred to as a "thermal asperity", while the defect on the disc is referred to as an "asperity". However, since one is indicative of the other, the two terms are frequently used interchangeably.
Disc asperities which are located in the factory during a defect scanning process can be recorded in a disc drive's primary defect list so that the drive does not store data at those locations. Known asperity detection techniques use sensors (such as MR sensors or piezoelectric sensors). Such known asperity detection techniques rely both on the flying characteristics of the heads and upon the thermal response from friction induced head/asperity contact. The energy of the impact or amplitude detected by an MR or other sensor is calibrated to determine the asperity characteristics such as height of the asperity. By calibrating the slope and duration of the resistance change waveform to a range of asperity heights and characteristics, the height of a particular asperity can be determined by detecting the momentary change in resistance of the sensor after contact.
However, the voltage signals corresponding to the impact of a sensor element with an asperity include components of noise, air bearing excitation, and other vibrations or excitations which may detract from the accuracy of calibrating the height of an asperity based upon the voltage signal from an MR sensor element or a piezoelectric sensor element after contact with the asperity.
Additionally, such devices require that the disc surface be scanned at various fly heights of the head so that various sizes of asperities can be detected to map the entire range of defects. As the speed of rotation of the disc is changed, the response of the specially-designed heads also changes. For example, if the speed is reduced, the energy of impact is reduced, thus making it more difficult to calibrate the defect size and height.